PHOENIX — About a dozen people carrying guns, including one with a military-style rifle, milled among protesters outside the convention center where President Barack Obama was giving a speech Monday — the latest incident in which protesters have openly displayed firearms near the president.

Gun-rights advocates say they're exercising their constitutional right to bear arms and protest, while those who argue for more gun control say it could be a disaster waiting to happen.

Phoenix police said the gun-toters at Monday's event, including the man carrying an AR-15 semi-automatic rifle slung over his shoulder, didn't need permits. No crimes were committed, and no one was arrested.

The man with the rifle declined to be identified but told The Arizona Republic that he was carrying the assault weapon because he could. "In Arizona, I still have some freedoms," he said.

Keeping peace on both sidesPhoenix police Detective J. Oliver, who monitored the man at the downtown protest, said police also wanted to make sure no one decided to harm him.

"Just by his presence and people seeing the rifle and people knowing the president was in town, it sparked a lot of emotions," Oliver said. "We were keeping peace on both ends."

Last week, during Obama's health care town hall in New Hampshire, a man carrying a sign reading "It is time to water the tree of liberty" stood outside with a pistol strapped to his leg.

"It's a political statement," he told The Boston Globe. "If you don't use your rights, then you lose your rights."

Police asked the man to move away from school property, but he was not arrested.

Beginning of a disturbing trend?Fred Solop, a Northern Arizona University political scientist, said the incidents in New Hampshire and Arizona could signal the beginning of a disturbing trend.

"When you start to bring guns to political rallies, it does layer on another level of concern and significance," Solop said. "It actually becomes quite scary for many people. It creates a chilling effect in the ability of our society to carry on honest communication."

He said he's never heard of someone bringing an assault weapon near a presidential event. "The larger the gun, the more menacing the situation," he said.

Phoenix was Obama's last stop on a four-day tour of western states, including Montana and Colorado.

Authorities in Montana said they received no reports of anyone carrying firearms during Obama's health care town hall near Bozeman on Friday. About 1,000 people both for and against Obama converged at a protest area near the Gallatin Field Airport hangar where the event took place. One person accused of disorderly conduct was detained and released, according to the Gallatin Airport Authority.

Heather Benjamin of Denver's Mesa County sheriff's department, the lead agency during Obama's visit there, said no one was arrested.

Arizona is an "open-carry" state, which means anyone legally allowed to have a firearm can carry it in public as long as it's visible. Only someone carrying a concealed weapon is required to have a permit.

Gun-control group: ‘This is craziness’Paul Helmke, president of the Washington, D.C.-based Brady Campaign to Prevent Gun Violence, said people should not be allowed to bring guns to events where Obama is.

"To me, this is craziness," he said. "When you bring a loaded gun, particularly a loaded assault rifle, to any political event, but particularly to one where the president is appearing, you're just making the situation dangerous for everyone."

Video: Protester brings gun to town hall protest
He said people who bring guns to presidential events are distracting the Secret Service and law enforcement from protecting the president. "The more guns we see at more events like this, there's more potential for something tragic happening," he said.

Secret Service spokesman Ed Donovan said armed demonstrators in open-carry states such as Arizona and New Hampshire have little impact on security plans for the president.

"In both cases, the subject was not entering our site or otherwise attempting to," Donovan said. "They were in a designated public viewing area. The main thing to know is that they would not have been allowed inside with a weapon."

Representatives of the National Rifle Association did not return calls for comment.